Electrooptic waveguide devices form an essential component of the emerging field of integrated optics, and are based on the phenomenon of electrooptics. Electrooptics is a property whereby materials change their refractive index upon the application of an electric field. This change in refractive index affects the way the material interacts with light. Electrooptics and electrooptic waveguide devices are described, for example, in Optical Integrated Circuits, by H. Nishihara et al. McGraw-Hill Book Company, New York, 1985, and in Integrated Optics: Theory and Technology, by R. G. Hunsperger, 2nd edition, Springer-Verlag, New York, 1985.
Electrooptic waveguide devices can be passive waveguide devices or functional waveguide devices. Some passive waveguides are optical beam-dividers, polarizers, and the like. Some functional waveguides are phase modulators, Mach-Zehnder modulators, and the like. Generally, electrooptic waveguides, or optical waveguides in short, consist of a transparent waveguiding core ("guiding layer") surrounded by a layer of transparent materials ("cladding layer"). Several general methods are utilized for the fabrication of optical waveguides.
In one method, optical waveguides are formed by applying a dielectric material to a substrate of lower refractive index.
In another method, optical waveguides are formed by selectively altering the refractive index of a bulk transparent material. One technique involves ion bombardment in which selected regions of different refractive index are provided by generating a molecular disorder pattern in a bulk matrix. In another technique, selected regions of different refractive index are either photo-induced in photo-sensitized polymeric materials such as poly(methyl methacrylate) as described in Applied Physics Letters, 16, 486 (1970), or electrically induced by diffusing a different index dopant into a transparent material.
Optical waveguides fabricated in GaAs/AlGa structures by laser-assisted etching have been reported in Integrated and Guided-Wave Optics, 1989 Technical Digest Series, 4, 64-67 (Optical Society of America).
Optical waveguides consist of an active guiding layer and a cladding layer as afore-mentioned, and optionally, additional layers. Among these layers, the guiding layer serves the important function of interacting with and affecting the propagation of light. Materials that form the guiding layer have been traditionally inorganic materials such as lithium niobate, potassium dihydrogen phosphate, ammonium dihydrogen phosphate, and the like. These are typically single crystal materials, and lack processing capabilities. In recent years, organic polymeric materials are being reported in increasing numbers that possess good processability due to their ability of being cast as films by well known methods, and can also serve as guiding layers. Such polymers typically possess the property of nonlinear optical activity, and hence are referred to as nonlinear optical polymers.
Nonlinear optical polymers contain nonlinear optical moieties as covalently linked part of polymer chains. Examples of such polymers are described in Nonlinear Optical Properties of Organic and Polymeric Materials, ed. D. J. Williams, ACS Symposium Series No. 233, American Chemical Society, Washington, D.C., 1983. The nonlinear optical moiety may be part of the polymer backbone, or it may be appended to the polymer backbone through intervening spacer groups. The latter are referred to as side chain nonlinear optical polymers. EP 89402476.9, for example, discloses nonlinear optical polymers where the nonlinear optical moiety forms part of the polymer backbone. U.S. Pat. Nos. 4,779,961; 4,801,670; 4,808,332; 4,865,430 and 4,913,844 disclose several side chain nonlinear optical polymers.
Nonlinearity of moieties is described in terms of second order nonlinearity, third order nonlinearity, and so on, with the corresponding unit values being referred to as second order nonlinear optical susceptibility, third order nonlinear optical susceptibility, and so on. Nonlinear optical moieties of polymers that are preferred as guiding layers in optical waveguide devices generally must possess acceptable second order nonlinear activity. These moieties are generally made up of conjugated .pi.-electron systems with an electron donating group such as an amine group, and an electron-acceptor group such as a nitro group forming either end of the conjugated .pi.-electron system.
Nonlinear optical polymers are generally cast as films on substrates by processes such as spin coating from a solution of the polymer in a solvent, spraying, Langmuir-Blodgett deposition, and the like. The substrate materials employed for electrooptic waveguide devices are generally inorganics such as silicon, GaAs, GaAlAs and the like. Silicon is particularly preferred as substrate material due to its ready availability in wafer form in a well-purified state, and the highly-developed state of its technology in integrated circuit and electronics industries. Wafers from silicon also have the advantage that they can be easily cleaved into minute chips carrying the individual devices.
The fabrication of electrooptic waveguide devices from nonlinear optical polymers, such as, for example, the polymers described in U.S. patents referred to above, typically involves the deposition and curing of a plurality of layers of films on the substrate. A typical polymeric electrooptic waveguide device fabrication comprises, for example, deposition of a polymeric film, a lower electrode layer, a lower cladding layer, active guiding layer, an upper cladding layer, and an upper electrode layer. These layers are successively deposited and cured, thus involving successive heating and cooling operations during the fabrication process. While the nonlinear optical polymers are generally considered to be relatively tough, and able to withstand the thermal energy that is needed to process them, it has been discovered that microscopic mechanical deterioration occurs during the fabrication, including, for example, cracking or crazing in the polymer layer or blistering (delamination) of the polymer layer from the inorganic substrate. Such deterioration has significant deleterious effects on the yields, utility and performance of the devices.
One cause of this problem is the difference in the coefficients of thermal expansion of the polymer material as compared to the substrate material. Due to this mismatch of the thermal expansion coefficients, a thermal stress is developed in the layers during the repeated heating and cooling operations in the fabrication process, thus resulting in the defects mentioned above.
Accordingly, it is an object of this invention to provide electrooptic waveguide devices with substantially reduced thermal stress in them.
It is another object of this invention to provide superior performing electrooptic waveguide devices.
It is a further object of this invention to provide an improved method for the fabrication of polymeric electrooptic waveguide devices.
Other objects and advantages of the present invention shall become apparent from the accompanying description and Examples.